Numerous studies have addressed the role of birth order on cognitive ability measures but to date little consensus has emerged. Most of the existing evidence of a negative relationship between birth order and child cognitive development and achievement is based on conventional regression analysis using cross-sectional data. In particular, no research has investigated the effect of birth order on the development of young children using large sibling samples. I propose to investigate the role of birth order on child development and scholastic achievement using better data and methods. I will also assess the role of ultimately completed family size and how it relates to the effect of birth order on child development. Specifically, I will investigate whether (1) children who are late in the birth order face elevated risks of adverse developmental and scholastic outcomes, (2) the birth order effect is causal or the result of spurious correlation, (3) there is an independent effect of ultimately completed family size on development, (4) the birth order and completed family size effects differ for minority and low-income families, (5) the effect of birth order on behavioral development differs from its effect on cognitive development, (6) the birth order, completed family size, and sibling spacing have long-term effects on scholastic attainment. In order to study these issues I use mother-child data from the National Longitudinal Survey of Youth 1979 (NLSY79) and data on the educational attainment of young adults from the Panel Study of Income Dynamics (PSID). These large nationally representative longitudinal surveys that over sample minorities provide good measures of the socio-economic conditions and family size during childhood and a rich set of measures of cognitive and behavioral development and scholastic achievement. The NLSY79 data are used to explore the effects of birth order on developmental outcomes during childhood. Longer-term effects of birth order on attainment are investigated using schooling data from the PSID. With larger and more representative samples, the proposed empirical models can properly identify the effect of birth order and complete family size effects on child achievements. [unreadable] [unreadable]